A commonly used electric circular saw has a saw blade which is rotatable at high speed and a switch trigger for activating a motor. In the non-operation state, if the saw blade is exposed, an accidental operation for the switch trigger will cause a serious safety accident. Generally, the saw blade of the electric circular saw is provided with a movable shield. In the operation state, as the electric circular saw moves relative to the workpiece, the movable shield enables the saw blade to be exposed so as to cut the workpiece; in the non-operation state, the movable shield covers the saw blade under the action of the biasing of an elastic member, thereby avoiding the safety risk caused by the accidental operation. However, there is also a problem: in the non-operation state, as the electric circular saw moves relative to the workpiece, the exposure of the saw blade may cause damage to the surface of the workpiece and affect the appearance of the workpiece.
Additionally, the movable shield cannot provide a safety protection if the operator needs to exchange the saw blade of the electric circular saw, thus it is necessary to provide a switch protection mechanism to the switch trigger, thereby avoiding the safety accident caused by the start-up of the motor due to the accidental operation. Generally, a micro switch is attached in series to a main switch of the electric circular saw, and the on-off of the main switch may be controlled by the on-off of the micro switch, thereby preventing the main switch from being triggered by the accidental operation. But, there is still a problem: if the micro switch is shorted and then the safety protection thereof is ineffective, when the people is unaware and operates it normally, a safety accident would occur.